What can fix this transmission problem? on 1996 Cadillac DeVille

I just got a car that has transmission problems with 135k on the car. If you put into (D) Drive, the transmission will quit engaging at around 45 mph. If I shift to gear position 3,2,1 the transmission seems to function normally.

I have been told that the transmission needs to be replaced, but I am skeptical. Any suggestions would be appreciated!

Check for trouble codes.If you have PO717 code (input speed sensor)you will not get you'r 4th gear.You do not have to pull the trans. Lower the engine/trans. cradle just far enough to get the side pan off through the left front wheel well.after removing the pan you will see the sensor /wires when replacing it you should get the distance specs between the contacts.Any way its a lot less expensive then removing the engine and tranny. Good Luck JRHardy